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If you thought you knew which foods you should and shouldn't be eating on a regular basis to stay healthy, then think again.

London nutritionist Rhiannon Lambert has exclusively told Femail Food&Drink the foods she would never buy - and many of them are products most of us would consider "healthy".

From fruit juice and low-fat yoghurt to agave syrup - often touted by clean eating bloggers as a "healthy" alternative to refined sugar - these are the foods that any self-respecting nutritionist would avoid putting in their bodies.

Agave syrup

A favourite of clean eating gurus such as "Deliciously" Ella Mills, Rhiannon says that agave syrup is in fact not as healthy as some bloggers would have you believe.

Agave is a cactus native to Latin America and the syrup is made from the pulp of the cactus leaf,' she says. 'The processing involved in its production destroys all of the health promoting properties of the agave plant.

"Agave syrup has a low Glycemic Index (GI) score - a measure of how quickly the sugar in a food enters your bloodstream - because of its low glucose content. But this is irrelevant as agave syrups are dangerously high in fructose content, often as high as 90 per cent.

"In this way, agave syrup is similar in composition to high fructose corn syrup, which is a processed sugar common in the US and is thought to be largely responsible for much of the country's obesity epidemic."

Low-fat yoghurt

Rhiannon says many low-fat yoghurts are as high in sugar as a dessert, despite often being marketed as a healthy option. Photo / 123RF

"Glucose from carbohydrate is also a form of sugar, and dried fruit to top it all off is a fast release of fructose sugars.

"Avoid these so-called healthy snack bars and go for something balanced with no extra sugars and added protein."

Protein bars

Rhiannon says she would never eat a protein bar that contained any artificial sweeteners, and explains why.

"These bars are labeled as a healthy, low-sugar, low-carb treat or dessert, with many on Instagram addicted to microwaving them and enjoying them like a warm brownie. But they contain sucralose, an artificial sweetener made from sugar in a multi-step chemical process where three hydrogen-oxygen groups are replaced with chlorine atoms.

"Research suggests sucralose raises blood sugar levels, making us hungry when levels crash - and that contributes to weight gain."

Instant noodles

Instant noodles may be much more dangerous for your health than you may realise, as Rhiannon says eating too much of them could lead to increased risk of heart disease. Photo / Brett Phibs

Instant noodles have never been branded as a health food, but if you've been telling yourself that they're a healthier snack than a chocolate bar, Rhiannon is here to shatter that illusion.

She explains: "Who can actually read all the hundreds of ingredients in these items? I couldn't even classify them as food. A study by The Journal of Nutrition shows that the consumption of instant noodles and pre-cooked noodle-based meals (that's anything you make by adding boiling water or heating in the microwave before eating) may increase the risk of cardiometabolic syndrome.

"That can in turn lead to heart disease, diabetes, or a stroke. Metabolic disease is also linked, with researchers suggesting that the increased risk is likely the result of the high calories, refined carbs, saturated fat, and sodium content typically found in instant noodles.

"Women in particular might be susceptible to these negative health effects because metabolic differences and hormones that affect women disproportionately. For instance, the chemical Bisphenol A (BPA), which is found in the styrofoam containers frequently used to package instant noodles, messes with the way that hormones signal messages within the body, particularly estrogen."

Tomato ketchup

You may not have regarded ketchup as particularly healthy or unhealthy but Rhiannon says she would never eat the condiment.

She explains: "Two ingredients of concern in ketchup are salt and sugar. Per tablespoon, ketchup contains four grams of sugar and 190 milligrams of sodium.

"Although four grams of sugar doesn't seem like a lot, much of it comes from added sugar, as opposed to the natural sugar found in tomatoes. The same goes for the added salt: consuming eight tablespoons of ketchup makes up your whole salt allowance for the entire day.

"Ketchup also typically includes high fructose corn syrup, natural flavoring, and tomato concentrate."

"Many fruit juices are actually little more than fruit-flavored sugar water," she explains.

"It is true that the juice contains some antioxidants and vitamin C, but this must be weighed against the large amount of liquid sugar.

"Eating actual fruit will give you benefits from fibre and antioxidants, but the cons of the sugar intake by drinking juice far outweigh the nutritional value.

"Be wary as concentrate means more processing and often, even more added sugar."

Cereals

Rhiannon says the only cereals she would consider eating are those which are oat-based - and says the worst culprits are those marketed as assisting with weight loss.

She comments: "The worst culprits are cereals marketed for weight loss or at children. These cereals lack protein, have added sugar and chemicals, use-refined carbohydrates and, coupled with milk, it's a sugar fest first thing in the morning.

"I actually view cereal as a dessert in a bowl - especially granola and crunchy nut varieties. The portion is also very small when you weigh it out and most of us eat double the amount required very easily."

A few companies now offer supposedly "healthy" biscuit options for breakfast now, as an alternative to energy bars.

But Rhiannon says she would never eat one, no matter how pressed for time she was.

"Biscuits for breakfast sound too good to be true, and they are," she says.

"... The calorie count is also high in comparison to many other breakfast substitutes, meaning these are probably one to have as a treat rather than on a daily basis. In fact, I would look at them in the same way as a normal biscuit."